If it was working, you have not put in too little yeast. The yeast feeds on the sugar and multiplies, the waste products of this are alcohol and CO2 (bubbles).
There are really only 3 things that will stop it.
1. If it gets too cold it will slow down to a point where you'll think it's stopped, warm it and off it goes again.
2.The yeast may have used up all the available sugar and will start to die off. This means you have as much alcohol as possible from that sugar and you'll get a dry wine.
3. If there is a lot of sugar in the wine the yeast will carry on working until a point where the amount of alcohol in the wine kills it off. You'll then get a sweet, strong wine.
As a guide, about 2lbs of sugar to the gallon usually gives a dry wine of "table wine" strength. If you're not sure taste a drop. If it's dry but too weak, add more sugar a bit at a time. If it's sweet, and not stopped because it's cold, then you will have a sweet wine and there's not a lot to remedy it except blending.
Hope this helps
Dave